Gas water heater draft hoods provide additional air for the combustion process gases, to be properly pulled out from the burner chamber (base of the water heater), into the water heater vent pipe and the chimney.
Mycelium is a block added in Minecraft the same time as the introduction of the Mushroom Biomes in Minecraft 1.9pre1. Mycelium can only be found and only can be found in Mushroom Biomes. Mycelium resembles dirt in that it can be broken with a shovel and it without a 'Silk Touch' enchanted tool it only drops dirt when broken. Mycelium also lets off a small particle effect the same as Bedrock.
The mushrooms collect sugars from the tree and, in turn, provide the tree with water and nutrients. The fruiting bodies of mycorrhizal mushrooms do not grow directly on the tree but instead emerge from the soil surrounding the tree; underground, the mycelia of the mushrooms are intertwined with the roots of the tree.